1. Field of the Invention
The present invention relates to a process for producing a very thin amorphous alloy strip suitable for applications such as saturable reactors, noise filters, choke coils, other inductors, various transformers and magnetic heads, and excellent in magnetic properties such as iron loss, magnetic permeability and squareness ratio, particularly in at a high frequency.
2. Description of the Prior Art
Thin strips of amorphous alloys are mainly produced by a method called a "single roll method" which comprises spraying a molten metal against a roll being rotated at a high speed to rapidly cool the metal. The single roll method is roughly classified into a planar flow casting method (hereinafter referred to as "PFC method") and a chill block melt spinning method (hereinafter referred to as "CBMS method"). The PFC method is characterized in that use is made of a slit nozzle and the space between the roll and the nozzle is limited to 0.03 to 1 mm to support a basin (a puddle) between the roll and nozzle (see Japanese Examined Patent Publication (Kokoku) No. 61-5820). This method is more suitable for the production of a thin strip having a more homogeneous sheet thickness and a larger width than a thin strip produced by the CBMS method that does not limit the space between the roll and the nozzle. A reduction in the thickness of a thin strip of an amorphous alloy to a very small value using the PFC method and CBMS method has been studied for the purpose of coping with an increase in the operating frequency of electronic equipment in recent years.
Yagi and Sawa et al. (see Japanese Unexamined Patent Publication Nos. 63-215348 and 3-90547 and page 6 of Material for the 26th Seminar of the 147th Committee on Amorphous Materials in the Japan Society for the Promotion of Science) disclose a process for producing a very thin amorphous alloy, characterized in that a very thin amorphous alloy strip is produced by the single roll method in an atmosphere evacuated close to vacuum. The production of a very thin amorphous alloy strip in the evacuated atmosphere causes the amount of gas entrapped between the chilling face of the roll and the molten metal to be reduced, which makes it possible to provide a thin amorphous alloy strip having a small sheet thickness and a smooth surface. The thin amorphous alloy strip thus obtained has excellent magnetic properties at a high frequency. Since, however, the production thereof in the above-described evacuated atmosphere is likely to give rise to occurrence of seizure of the molten metal on the roll, the yield is low. Further, the size of facilities needs to be large, so that the above-described process is disadvantageous also in profitability.
Davor Pavuna (see Journal of Materials Science, 16 (1981) 2419-2433) discloses that, in the CBMS method, helium (He) is blown against the puddle to stabilize the puddle. In this case, He serves to reduce the instability of the puddle observed in the CBMS method. In this method, the resultant thin strip has an average sheet thickness of 24.6 .mu.m at the smallest and a smoothness of 4 .mu.m in terms of the variation of the thickness. The sheet thickness and smoothness are equivalent to a thin strip provided by the conventional PFC method, so that the thin strip is unsatisfactory as a very thin amorphous strip having a smooth surface.
Howard Host Riverman (see Japanese Examined Patent Publication (Kokoku) No. 2-18665) discloses a process for producing a metallic strip by the single roll method or the like, characterized in that an inert gas (N.sub.2, He, Ne, Ar, Kr, Xe or their mixture) heated to reduce the density is blown in a rapid cooling zone to provide a thin amorphous alloy strip having a smooth surface. Although this process needs no special low pressure vessel, the smoothing effect cannot be attained without reducing the density of the inert gas to be sprayed. Further, this publication describes that "In order to provide a desired low density atmosphere, the gas is heated to at least 800 K, preferably 1300 K." That is, the use of a gas at 800 K or above is indispensable.
Further, Wen-Kuan Wang et al. (see International Journal of Rapid Solidification, 6 (1991) 285-295) have succeeded in providing a thin strip having a smooth surface portion free from an air pocket by heating a gas upstream of the puddle with an arc plasma in the PFC method. Since, however, the roll surface becomes very roughened, this process is unsuitable for use in casting for a long period of time. Further, since the surface of the roll reaches a high temperature, difficulties will be encountered in providing rapid cooling conditions that enable the thin amorphous alloy strip to be stably produced for a long period of time.
As described above, a very thin amorphous strip has hitherto been provided by using an evacuated atmosphere or blowing a heated inert gas. For these purposes, it is necessary to use a vessel for regulating the atmosphere and an apparatus for heating a blowing gas, which is unfavorable from the viewpoint of profitability.
An object of the present invention is to 30 provide a process for producing a thin amorphous alloy strip that can cope with an increase in operating frequency and enables a thin amorphous alloy strip having a very small thickness and a smooth surface free from significant uneven portions and pores to be easily produced, without use of any vessel for regulating the atmosphere and an apparatus for heating a blowing gas, at a low cost.